1. Field of the Invention
The present invention relates to a semiconductor memory device and a method of fabricating the same. More particularly, the present invention relates to a semiconductor memory device including an improved heating unit for heating a storage/memory node made of a phase change material, and a method of fabricating the same.
2. Description of the Related Art
Ovonic unified memories (OUMs) use phase change materials as a data storing portion instead of using capacitors as in general dynamic random access memories (DRAMs). An OUM is a memory device that writes/reproduces information based on a principle that an electric resistance is changed according to a phase change of a chalcogenide material. Chalcogenide materials have been used in re-writable CDs, DVDs, and the like. When a chalcogenide material is transformed from a crystalline state into an amorphous state, the resistance of the chalcogenide material increases approximately 100 times. The chalcogenide material undergoes a phase change, which is controlled by a heating temperature and a cooling time. Accordingly, when the chalcogenide material is in a crystalline state, it has a low resistance and acts as a conductive material. When the chalcogenide material is in an amorphous state, however, it has a high resistance and acts as a resistive material.
FIG. 1 illustrates a schematic cross-sectional view of a conventional OUM.
Referring to FIG. 1, a lower electrode 102 having a tip portion 114 is formed on a substrate 100. A chalcogenide material layer 128 and an upper electrode 122 are deposited over the tip portion 114 of the lower electrode 102. A conductive barrier layer 120 is interposed between the chalcogenide material layer 128 and the lower electrode 102 to prevent diffusion between the chalcogenide material layer 128 and the lower electrode 102. The upper electrode 122 is connected to a grid interconnection layer 126 to receive electric power from an outside source. In FIG. 1, reference numerals 116 and 124 denote insulation layers.
The chalcogenide material is formed of a three-phase system, namely, germanium (Ge)-Tellurium (Te)-antimony (Sb). After a predetermined current is applied to the chalcogenide material, the chalcogenide material is transformed to an amorphous state according to a heat capacity applied thereto. Then, the resistance of the chalcogenide material is changed according to a cooling time. More specifically, when atoms of the chalcogenide material are slowly cooled in an amorphous state, they become crystalline and act as a conductive material. However, when the atoms are rapidly cooled, they become amorphous and exhibit a high resistance. Such a resistance change can be expressed in a binary code and functions as a memory factor.
Therefore, a binary code “1” or “0” can be written in a selected cell of the OUM through the phase change. Further, the written binary code “1” or “0” can be read by reading the resistance of the programmable region.
An OUM has several advantages in that abrasion does not occur even though the memory may be read many times, a very small amount of voltage is necessary, and a compatibility with existing design environments is high. Furthermore, since the OUM can be used about one billion times, i.e., one thousand million times, it can easily substitute for existing high capacity storage.
There is a disadvantage, however, in that to transform the chalcogenide material layer 128 into an amorphous state in the conventional OUM structure as described above, the chalcogenide material layer 128 should be heated to above 600° C. Therefore, a large amount of current is required to be applied to the lower electrode 102.